Mo promoted Ni-Al2O3 co-precipitated catalysts for green diesel production

Abstract

Three Mo promoted Ni-Al2O3 co-precipitation catalysts with practically the same composition (49–52%wtNi, 6-7%wtMo) were prepared using three different co-precipitation modes: co-precipitation at room temperature using ammonia as precipitating agent as well as co-precipitation at higher temperature (110 °C) using ammonia or urea as precipitating agent. The corresponding un-promoted catalysts were also synthesized for comparison. The catalysts were exhaustively characterized using various techniques and evaluated for the selective deoxygenation (SDO) of natural triglycerides using two different feedstocks: sunflower oil (SO) and waste cooked oil (WCO). The catalytic tests were performed under solvent free conditions in a semi-batch reactor at 310 °C, hydrogen pressure 40 bar and very high reactant volume to catalysts mass ratio (100 mL/1 g).The promoting action of the Mo(VI) and Mo(IV) well dispersed oxidic phases, was demonstrated in all cases. This was attributed to the impressive decrease in the size of the nickel nanoparticles and to the inhibition of the formation of the catalytically inactive nickel aluminate. Moreover, the Mo (VI) and Mo(IV) oxidic phases affect the network of the SDO favoring the hydrodeoxygenation of the intermediate alcohols with respect to the decarbonylation of intermediate aldehydes.The catalyst prepared at high co-precipitation temperature using ammonia as precipitating agent was proved to be the most efficient for the SDO of SO. An almost complete transformation of SO into n-C15, n-C16, n-C17, n-C18 (green diesel: 97% of liquid products) was obtained under the above mentioned conditions. This catalyst was proved to be very stable. The catalyst prepared at room co-precipitation temperature using ammonia as precipitating agent was proved to be the most efficient for the SDO of WCO (green diesel: 76% of liquid products). Its improved efficiency with respect to the other Mo promoted catalysts was attributed to its smaller pore size which prohibits the blockage of active sites located inside the corresponding pores by bulky compounds present in WCO.